The present disclosure relates generally to a method and system for measuring cell stabilities in a real SRAM product with in-situ parametric diagnosis.
Memory cells of an SRAM product often require testing and diagnosis. However, the conventional methods for diagnosing SRAM are quite troublesome, costly, and inaccurate. Some of these conventional methods include bit mapping, node probing, and test structure measuring. The bit mapping method builds a map of passed and failed bits that correspond to their physical locations. While the method can provide statistical data of proximity effects or IR drops in power and ground buses, it cannot provide a noise margin for each bit cell. Node probing is another method for diagnosing SRAM by probing internal nodes such as bit lines, data lines, or sense amplifier outputs. As technology improves, metal widths within circuitry shrink. As such, probing by tips becomes increasingly difficult. Probing small areas require e-beams which can be very expensive and time consuming. Test structure measuring requires device nodes in an SRAM cell to be brought out to pads in test structures so that such node voltage can be measured, but the test environment tends to deviate from the real SRAM environment. The result is the inability to provide yield enhancement information of each bit cells. Even though all these methods are useful, however, there is still room for improvement on efficiency and accuracy.
It is always desirable to have an efficient, accurate, and low cost system for SRAM cells measurements.